The ALICE Collaboration reports its first LHC Run 3 measurements of charged-particle pseudorapidity density at midrapidity in Pb-Pb collisions at a centre-of-mass energy per nucleon pair of $\sqrt{s_{\mathrm{NN}}}=5.36$ TeV. Particle multiplicity in high-energy collisions characterises the system geometry, constrains particle-production mechanisms, and is used to estimate initial energy density. Multiplicity also acts as a reference for subsequent measurements as a function of centrality. In this letter, for the first time, charged particles are reconstructed using the upgraded ALICE Inner Tracking System and Time Projection Chamber, while the collision centrality is determined by measuring charged-particle multiplicities with the Fast Interaction Trigger system. Pseudorapidity density, ${\rm d}N_{\rm ch}/{\rm d}\eta$, is presented, averaged over events, for various centrality classes. Results are shown as a function of pseudorapidity and the average number of participating nucleons ($\langle N_{\mathrm{part}}\rangle$) in the collision. The average charged-particle pseudorapidity density ($\langle {\rm d}N_{\rm ch}/{\rm d}\eta \rangle$) at midrapidity ($|\eta|<~0.5$) is 2047 $\pm$ 54 for the 5% most central collisions. The value of $\langle {\rm d}N_{\rm ch}/{\rm d}\eta \rangle$ normalised to $\langle N_{\mathrm{part}}\rangle/2$ as a function of $\sqrt{s_{\mathrm{NN}}}$ follows the trend established in previous measurements in heavy-ion collisions. Theoretical models based on mechanisms for particle production in nuclear collisions that involve the formation of quark-gluon plasma medium and models based on individual nucleon-nucleon interactions are compared to the data.
Submitted to: PLB
e-Print: arXiv:2504.02505 | PDF | inSPIRE
CERN-EP-2025-073
Figure group
Figure 1
Charged-particle pseudorapidity density at midrapidity for ten centrality classes in Pb--Pb collisions at $\sqrt{s_{\mathrm{NN}}}=5.36$ TeV. Grey and open boxes show the uncorrelated and correlated systematic uncertainties over pseudorapidity, respectively. Statistical uncertainties are smaller than the marker size. |  |
Figure 2
Comparison of charged-particle pseudorapidity density measured in 0--80\% and 0--5\% Pb--Pb collisions over $|\eta|1$ with similar measurements from the CMS experiment in $|\eta|2.6$. For ALICE data, grey and open boxes show the uncorrelated and correlated systematic uncertainties over pseudorapidity, respectively. Statistical uncertainties are smaller than the marker size. For CMS data, the vertical line denotes the total systematic uncertainties. |  |
Figure 3
Values of (2/$\langle N_{\mathrm{part}} \rangle$)\avdndeta for central Pb--Pb and Au--Au collisions as a function of $\sqrt{s_{\mathrm{NN}}}$. The $\sqrt{s_{\mathrm{NN}}}$ dependence of the heavy-ion collision data is well described by the function $s^{0.156}_{\mathrm{NN}}$ (solid line). The shaded band represents the uncertainty on the extracted power-law dependency. The central Pb--Pb measurements from ATLAS at $\sqrt{s_{\mathrm{NN}}}=2.76$ TeV and from CMS at $\sqrt{s_{\mathrm{NN}}}=2.76$ and 5.36 TeV have been shifted horizontally for clarity. |  |
Figure 4
Dependence of (2/$\langle N_{\mathrm{part}} \rangle$)\avdndeta on the average number of participating nucleons for Pb--Pb collisions at $\sqrt{s_{\mathrm{NN}}}=5.36$ TeV. Grey bands (open boxes) show the uncorrelated (correlated) uncertainties over centrality. Statistical uncertainties are within the marker size. Data from Pb--Pb collisions at $\sqrt{s_{\mathrm{NN}}}=2.76$ TeV scaled by 1.27 and $\sqrt{s_{\mathrm{NN}}}=5.02$ TeV scaled by 1.06 are shown for comparison. |  |
